Steam generator

ABSTRACT

In a steam generator having a steam generator chamber into which heating elements are installed, a guide casing surrounds the elements and has an open bottom and a top cover plate. A series of steam separators are provided above the top cover plate and are completely encircled by an annular overflow plate arranged on the top cover plate. The overflow plate has a brim which is located above the water outlets of the steam separators and it also contains a plurality of holes that allow only about half of the return water to flow off.

United States Patent I 1 u Meier July 15, 1975 STEAM GENERATOR 3.253.999 5/l966 Wcismann 5. 55/343 x 3507,09) 4 [970 M h' ll I22 488 X Inventor: Franz Meier, Oberhzwsen. Germany 3335569 i iz fl u [2/2/34 X [73] Assignee: Gutehofl'nungshutte Sterkrade AG,

Germany Primary ExuminerKenneth W. Sprague I Attorney, Agent, or Firm-Toren. McGeady and [22] Filed. Nov. 30, 1973 Stanger [21] App]. No: 420,564

[ ABSTRACT [30] Foreign Application Priority Data In a steam generator having a steam generator cham- Dec Zfi 1972 Germany 2259076 her into which heating elements are installed, a guide casing surrounds the elements and has an open bottom [52] us CL H 122/34; 122/491. 55/348 and a top cover plate. A series of steam separators are 51 Int. Cl. F22]; 37/26 Provided above the top Cover Plate and are completely 58 Field of Search 122/32, 34, 488. 491; encircled by an annular Overflow Plate arranged on 55/346 348 355v 455 456. 457 the top cover plate. The overflow plate has a brim which is located above the water outlets of the steam [56] Reierences Cited separators and it also contains a plurality of holes that UNITED STATES PATENTS allow only about half of the return water to flow off.

315mm 5/1966 Gleason 55/455 x 5 Claims, 2 Drawing Figures SHHT WIN

FIG. 2

STEAM GENERATOR The present invention concerns a steam generator having a steam generation chamber into which heating tubes are installed which are encased in a guide casing open at the bottom and having a cover plate at the top with steam separators and an annular overflow plate arranged on the top cover plate.

Steam generators of this kind are known. for instance. from DTGbm (German registered design) 7,128,822 and DT-OS (German first notice of disclosure) l.4Ul,()l4. Previously. it was considered suffcient in steam generators if on the customary top cover plate either only part of the steam separators were en circled by an overflow plate not incorporating any drain holes of specific dimensions. or if the water outlets of long separators were generally so arranged that the water outlet from the separators was located immediately below the steam generators water level when in operation.

As in conformity with DT-Gbm 7.l28.822 the overflow plate encircled only part of the steam separators and since. moreover. no drain holes at all were provided. improvement of the separation efficiency was only small when compared with older designs having no overflow plate.

As per DT-OS l.4()l.0l4 the hot steam/liquid mixture is not immediately conducted axially into a series of steam separators but first into a primary chamber shaped as a hexagonal regular prism consisting of shell and cover. In this design requiring a high expenditure it is only from this intermediate element that radial outlets extend to six swirl chambers comprising an upper and a lower portion the latter being the efflux part. Although the swirl chambers are so dimensioned that the normal liquid level is located in the upper the chambers this design is very complicated.

With still other steam generators the water level must be exactly maintained at a specific height. If the level falls by only a small amount from the specified height proper functioning of the separators is no longer guaranteed. Despite the arrangement of a steam-separating siphon in the water outlet of the separators steam will escape together with the water if the water level in the steam generator falls by a specific amount.

The presence ofsteam in the water returned involves grave disadvantages. Any steam present reduces the weight of the water column. This may lead to a breakdown of natural water circulation in the steam generator. In such circumstances there is the danger of steam generation becoming unstable by vibrations being produced in the steam generator. In the case of boilingwater reactors with forced circulation any steam in the water returned will reduce the admissible heatingsurface load capacity which may entail cavitation in the pumps.

Thus, the task to be solved by the present invention is to ensure proper functioning of the steam separators if the water level in the generator falls eliminate the danger of steam generation becoming unstable; to improve the separation efficiency of the separators by a marked degree; and to significantly simplify the design of these steam generators.

In the present invention. referring to steam generators as specified at the beginning, this task is accomplished by providing an overflow plate encircling all steam separators, and the brim of which is located above the water outlet of the steam separators with drain holes provided at the base of the overflow plate. the total cross-sectional area of which is so dimensioned that about half of the water separated by the separators flows off.

When arranged in conformity with the present invention the overflow plate together with the top cover plate form a sort of water seal which is constantly filled with return water during operation even if the water level in the steam generator falls. On account of the strikingly simple way of arranging the overflow plate the requirement of a narrow water-level control range can be abandoned. The control equipment otherwise required would demand a high expenditure. In the steam generator according to the present invention the water level may fall by a considerable amount without entailing any of the above described adverse consequences. Even in such circumstances no. or at the most only a negligible quantity of. steam would escape through the water outlet of the separators.

With the steam generator specified the water clearing the overflow plate will be returned into the steam generation chamber. It is more favourable for the return water to enter the chamber below the normal water level. According to the present invention drain holes will be provided for the return water at the base ofthe overflow plate. the cross-sectional area of which is rated for about half of the water quantity flowing through. This outlet-area rating means that if the water supply from the freshwater feed pipe fails completely the return water dammed up by the overflow plate ill only flow off slowly so that even in such adverse circumstances no steam can escape for at least a certain period.

The present invention will be exemplified below by reference to the following Figures:

FIG. I is a schematic longitudinal section of a steam generator incorporating the features of the present in vention.

FIG. 2 shows the features of the present invention on a larger scale.

The following example refers to the use of the present invention in a U-tube heat exchanger with a vertical tube bundle. The steam generator comprises a cylindri cal casing 1 into which a bundle of hairpin-shaped heating tubes 2 is installed (U-tubes). The tubes 2 are inserted in the tube sheet 3 so as to be absolutely tight. Under the tube sheet. collectors 4 and 5 are arranged. The heating agent enters collector 4, passes through the heating tubes 2 and is then led off through collector 5.

The heating-tube bundle 2 is encased in a cylindrical plate shell 6 open at the bottom and arranged at a certain distance from the inner wall of the casing I and the tube sheet 3. Above the heating tubes 2 the shell 6 is sealed off by a top cover plate 7 on which previously known steam separators 8 operating by the centrifugal principle are mounted.

Feed water is supplied to the steam generator through piping 9. The water flows through the annular clearance 10 between shell 6 and easing I, horizontally enters the heating-tube bundle 2 through tube sheet 3 and rises between these tubes. While rising the water is heated to boiling temperature by heat transfer and finally vaporized. The steam generated leaves the heating-tube bundle 2 as a mixture with the nonvaporized water entrained. and then enters the steam separators 8 in which pre-drying takes place. The pre-clried steam escapes through outlets 11 by imparting rotary motion to the mixture whereas the water obtairied in the" separators leaves the separators through water outlets 12. A siphon-type safety arrangement 13 provided in water outlet 12 of the separators 8 is to prevent the escape of steam together with the water. Steam will escape despite this safety arrangement if the water level in the steam generator falls too much below the normal height 14. For this reason. said water level would have to be maintained within narrow tolerance limits.

in order to achieve a wide water-level control range and. also. to prevent steam from leaving the separators through the water outlet an overflow plate 15 is arranged at the edge of the top cover plate 7. This overflow plate encircles the steam separators 8 as a ring and has its brim 16 located at a higher level than the water outlet 12 of the separators 8. Together with the top cover plate 7 the overflow plate 15 forms a sort of water seal which is constantly filled with water during operation. The existing water level prevents steam from escaping even if the water level in the steam generator falls markedly below the normal height 14.

Drain holes [8 are provided below normal water level at the base 17 of the overflow plate. The crosssectional area of these holes is smaller than would be required to ensure that all of the water returned flows off. Preferably. the holes are rated for half of the return water quantity to be expected.

In addition to the type of steam generator specified above the present invention can also be used in steam generators of a different design, for instance in units equipped with a straight-tube bundle. Further. the invention can be employed in boiling'water reactors. in such a case the heating elements are not U-shaped tubes through which a heating agent flows but the fuel elements of the nuclear reactor.

What we claim is:

l. A steam generator comprising an upwardly extending shell having an open bottom and a cover plate at the top. a casing laterally enclosing said shell. a bundle of heating elements within said shell for converting water into steam and the space within said shell containing said heating elements forming a steam generation chamber, a plurality of steam separators having water outlets located above said top plate and communicating with the steam generation chamber, an overflow plate mounted on and extending upwardly from said top plate on said top cover plate, said overflow plate laterally encircling all the steam separators and having a brim above the water outlets from said steam separators and said overflow plate having drain holes therethrough, situated at the base of the overflow plate for allowing a certain fraction of the water separated by said separators to flow off.

2. A steam generator according to claim 1 wherein said heating elements are U-shaped tubes.

3. A steam generator according to claim 1 wherein said heating elements are straight tubes.

4. A steam generator according to claim 1 wherein said heating elements are fuel elements of a nuclear reactor.

5. A steam generator according to claim 1 wherein said drain holes are located below the normal water level in said separators. 

1. A steam generator comprising an upwardly extending shell having an open bottom and a cover plate at the top, a casing laterally enclosing said shell, a bundle of heating elements within said shell for converting water into steam and the space within said shell containing said heating elements forming a steam generation chamber, a plurality of steam separators having water outlets located above said top plate and communicating with the steam generation chamber, an overflow plate mounted on and extending upwardly from said top plate on said top cover plate, said overflow plate laterally encircling all the steam separators and having a brim above the water outlets from said steam separators and said overflow plate having drain holes therethrough, situated at the base of the overflow plate for allowing a certain fraction of the water separated by said separators to flow off.
 2. A steam generator according to claim 1 wherein said heating elements are U-shaped tubes.
 3. A steam generator according to claim 1 wherein said heating elements are straight tubes.
 4. A steam generator according to claim 1 wherein said heating elements are fuel elements of a nuclear reactor.
 5. A steam generator according to claim 1 wherein said drain holes are located below the normal water level in said separators. 